Welding two relatively rotatable work pieces together at a common interface where the two work pieces are axially engaged under pressure at their interface is known in the art. The technique of spin welding two plastic pieces together has been known and successfully used to secure two relatively rigid plastic pieces together. It is also known in the art to weld a rigid plastic piece to a thin-walled, flexible plastic tube. U.S. Pat. No. 3,690,088 to Anderson for "Method of Packaging," for example, discloses a method of welding a relatively rigid plastic member to a flexible, tubular plastic member.
Alternative methods for spin welding thermoplastic work pieces together are discussed in U.S. Pat. No. 4,090,898 to Tuskos for "Methods and Apparatus for Spin Welding Thermoplastic Work Pieces". One method involves placing the work pieces into separate fixtures, rotating one of the fixtures and driving the work pieces into contact to effect the weld. The other method disclosed in Tuskos uses a driven work piece mounted to a stationary work piece. A rotating drive fixture is then used to grip, axially drive and spin the driven work piece to effect a weld.
The known prior art methods of spin welding thermoplastic pieces together are uniformly unsuccessful when attempting to weld one soft, flexible thermoplastic piece to another soft, flexible thermoplastic piece. Heretofore the only methods available for joining two soft, flexible thermoplastic pieces together have been by using adhesives or solvent adhesives. The use of adhesives or solvent adhesives to connect two thermoplastic pieces together is unacceptable where highly reliable connections are required. Solvent adhesives are also inconvenient to use and store and are thought by some to be undesirable.
In certain medical applications, flexible plastic containers, particularly polyvinyl chloride (PVC) bags, have membrane tubes connected in telescoping relationship with ports on the container. For example, Travenol Laboratories, Inc., manufactures a PVC two liter peritoneal dialysis solution bag which has two ports with membrane access and injection site tubes joined to the ports in telescoping relation. Conventional manufacture uses a solvent adhesive (cyclohexanone) to connect tubes to ports. It would be desirable to obtain a more reliable connection of tubes to ports while eliminating the use of solvent adhesives.
Spin welding is an alternative, but it heretofore has proved difficult to spin weld together two relatively soft and flexible plastic tubes (especially PVC). Relatively soft plastic tubes are generally those with durometers no greater than 30. Nevertheless, if a satisfactory spin welding method were found, generated particulate matter--a consequence of spin welding processes in general--would need to be minimized or eliminated if medical uses were contemplated for the plastic pieces.
By this invention, a method and an apparatus for spin welding together soft and flexible plastic tubes in telescoping relationship are provided eliminating the need to use solvent adhesives. Generated particulate plastic is minimized or eliminated by controlling the number of revolutions of one plastic piece relative to the other during a specified time, controlling the area of the welded interface, and controlling any free tip length of the inner tube beyond the welded interface.